The present invention relates to a process for producing a micro Fresnel lens (also known as a Fresnel zone plate) by exposure to actinic radiation such as electron beam, laser light or UV rays, and by subsequent development.
The micro Fresnel lens is a non-classical lens that is designed to have the optical properties such as light concentration by making use of diffraction. As shown in FIGS. 1 to 3, the micro Fresnel lens has concentric rings on a flat surface, and each ring has a rectangular cross section 1 or a sawtoothed cross section 2. A micro Fresnel lens having sawtoothed concentric rings 2 has a higher light concentration efficiency than a lens using rectangular concentric rings 1.
The conventional method of fabricating the micro Fresnel lens proceeds in the following manner. As shown in FIG. 4, a transparent blank such as a glass or acrylic resin plate that has been polished to a flat surface is coated with a resist layer 4, thereby providing a substrate A. An electron beam 5 is focused on the resist layer 4 and deflected in a circular pattern to scan the resist layer 4 for exposure.
Ideally, a sawtoothed setback 2 formed on the micro Fresnel lens should have the shape shown in FIG. 5(A), and this idealized shape can only be obtained by repeating the application of electron beam 5 to the same area of the resist layer. To this end, as shown in FIG. 5(B), the area corresponding to the highest point a of the sawtooth is scanned once, the area corresponding to the second highest point b is scanned twice, the area corresponding to point c is scanned three times, and the area corresponding to the lowest point d is given four scannings. By applying varying exposures or doses of electron beam to different areas, the thickness of resist layer 6 that is left intact after development can be varied in the respective areas.
In actuality, owing to the difference in intensity across an electron beam spot and the difference in exposure width, the setback remaining after development assumes the shape shown in FIG. 5(C) which is less like a staircase but more like the ideal sawtoothed shape.
In the conventional fabrication method using an electron beam as actinic radiation which describes a circularly deflected scanning pattern, a very large deflection angle cannot be employed and the widest area that can be exposed is only 1 to 2 millimeters across. It is therefore impossible to prepare a pattern of concentric rings for a large-diameter lens. Another disadvantage occurs when a plurality of patterns must be formed on a single substrate. After one pattern is formed by circularly deflected scanning, the substrate is moved a predetermined distance for receiving the scanning radiation to form the next pattern. This step-and-repeat process is unable to effect continuous movement of the substrate and requires a prolonged time in exposing the entire area of the resist layer. Furthermore, the circularly deflected scanning is not highly adaptive to the current practice of using a modified electron microscope as an electron beam scanner.